The first time, I knitted it a bit too long and so I have ripped it back and am prepared to start the ribbing and brim again.

The ribbing is k1p1 ribbing on size 5 needles.

My gauge is off and the hat is coming out too big. Is there anything I can do with the ribbing to make it smaller? Can I use even smaller needles? How about doing a few decreases in the round before the ribbing?

I think either of your ideas would work. I think you'd have a bit more control on the size by decreasing rather than smaller needles, since you won't be able to tell right away how much smaller the difference in needles will make it.